Springback, improvement of its predictability: Literature study report

The deep drawing process is commonly used to manufacture sheet metal products. During the process initially curved or flat blank material is clamped between the die and the blankholder. When the punch is pushed into the die cavity, the blank is plastically deformed and the specific shape of the punch and the die is transferred to it. After the tools are removed, the elastically-driven change of the product shape, or so-called springback, occurs. This phenomenon results into the deviation of the obtained product shape from the design specification and can be the major cause of assembly problems. In sheet metal forming the quality of the final product depends on the proper tools’ design, choice of the blank material, blankholder force, lubrication and some other process parameters. To manufacture a product with the desired shape and performance an extensive knowledge about the influence of various parameters is needed. In order to establish this knowledge base, experimental try-outs or numerical simulations are used. Finite element simulation of sheet metal forming is a powerful tool, which allows to test any modifications of the deep drawing process parameters, prior to the actual tools manufacturing. calculations can be made to predict and compensate for springback and the numerical simulations can be repeated as often as necessary until the product with the desired shape is produced. Currently the numerical analysis is not able to accurately predict the springback of a formed product. There is always a discrepancy between the level of springback obtained in simulations and reality, especially for the products with complicated geometry. The objective of this literature study is to understand the springback phenomenon and to ascertain the reasons of its inaccurate numerical prediction. Development of algorithms for improved numerical prediction of springback after trimming operation is a subject of a separate research [1] and is not dealt with in this study. One of the reasons for poor springback prediction is that this phenomenon is not accurately represented in finite element formulations. Various assumptions of material behaviour - constant elastic properties during forming, simplified elastic-plastic anisotropy and workhardening - introduce the large modelling error. In addition, the accuracy of the springback prediction is affected significantly by the quality of simulation of the forming operation. Chosen contact algorithms, the method of unloading, the time integration scheme, the element types and the level of discretization can be other reasons for significant deviation of the numerically predicted springback from that observed in real practice. Furthermore, an analyst plays an important role and substantial discrepancy of the springback results may be caused by unexperienced users.